In normal types of time-of-flight mass spectrometers, a specific amount of kinetic energy is imparted to ions derived from a sample component to make those ions fly a specific distance in a spatial area. The period of time required for their flight is measured, and the mass-to-charge ratio of each ion is calculated from its time of flight. Therefore, if there is a variation in the position of the ions or in the amount of initial energy of the ions at the time when the ions are accelerated and begin to fly, a variation in the time of flight of the ions having the same mass-to-charge ratio occurs, which leads to a deterioration in the mass-resolving power or mass accuracy. As a technique for solving such a problem, an orthogonal acceleration time-of-flight mass spectrometer, which accelerates ions into the flight space in a direction orthogonal to the incident direction of the ion beam, has been commonly known (this device is hereinafter appropriately abbreviated as the “OA-TOFMS”).
As just described, the OA-TOFMS is configured to accelerate ions in a pulsed fashion in the direction orthogonal to the direction in which a beam of ions derived from a sample component is initially introduced. Such a configuration allows the device to be combined with various types of ion sources which ionize components contained in a continuously introduced sample, such as an atmospheric pressure ion source (e.g. electrospray ion source) or electron ionization source. In recent years, the so-called “Q-TOF mass spectrometer” has also been widely used for structural analyses of compounds or similar purposes. In this device, the OA-TOFMS is combined with a quadrupole mass filter for selecting an ion having a specific mass-to-charge ratio from ions derived from a sample component as well as a collision cell for dissociating the selected ion by collision-induced dissociation (CID).
In the Q-TOF mass spectrometer, since CID gas is continuously or intermittently introduced into the collision cell, the gas pressure within the collision cell becomes comparatively high. Therefore, when ions having various mass-to-charge ratios exit from the collision cell, all ions have already been sufficiently cooled and have approximately equal amounts of kinetic energy. Accordingly, an ion with a smaller mass-to-charge ratio has a higher speed when arriving at the orthogonal accelerator in the OA-TOFMS. This causes a problem related to the duty cycle in the Q-TOF mass spectrometer, as will be hereinafter described (see Non Patent Literature 1).
FIG. 12 is a schematic diagram of an ion optical system including the sections from the collision cell to the orthogonal accelerator in a conventional Q-TOF mass spectrometer. Consider the case where various ions which have been sufficiently cooled within the collision cell 13 as described earlier are introduced through an ion transport optical system 14 (which is an electrostatic lens electrode) into the orthogonal accelerator 16 along the X-axis direction. The orthogonal accelerator 16 includes a plate-shaped push-out electrode 161 and grid-shaped extraction electrodes 162. A pulsed acceleration voltage is applied to those electrodes at constant frequency f, whereby the ions introduced into the orthogonal accelerator 16 are ejected toward the flight space (not shown) in the Z-axis direction. The ions to be ejected from the orthogonal accelerator 16 by this operation are ions which are present within a range having length L along the incident direction of the ions into the orthogonal accelerator 16 (X-axis direction). This range corresponds to the opening of the extraction electrodes 162. Ions introduced into the orthogonal accelerator 16 within the period of time (1/f) from one ejection of the ions to the next ejection of the ions directly pass through the orthogonal accelerator 16 and are eventually wasted.
Letting v denote the speed of an ion entering the orthogonal accelerator 16, the use efficiency of the ion, i.e. duty cycle ε, is defined as follows:ε=fL/v 
As noted earlier, the speed v of an ion depends on the mass-to-charge ratio of the ion. Therefore, the smaller the mass-to-charge ratio of the ion is, the lower the duty cycle becomes, which means that the amount of ions to be subjected to the analysis decreases and the detection sensitivity becomes lower.
To avoid this problem, a TOFMS described in Patent Literature 1 employs a method in which ions are temporarily accumulated within the collision cell 13, and the ions in the form of a mass (bunch) are discharged into the orthogonal accelerator 16 in a synchronized fashion with the ion-ejecting pulse in the orthogonal accelerator 16.
A specific description of this operation is as follows: A high voltage having the same polarity as the ions is applied to the exit lens electrode 132 of the collision cell 13 to block the ions and temporarily accumulate the ions within the collision cell 13. At a later point in time, the voltage applied to the exit lens electrode 132 is decreased, whereupon the ions compressed into a bunch-like form are discharged. After the passage of a specific length of delay time from the point in time where the voltage applied to the exit lens electrode 132 was decreased, an acceleration voltage is applied to the push-out electrode 161 and other related elements, whereby the bunch of ions discharged from the collision cell 13 are ejected into the flight space. Thus, in this TOFMS, ions which have been introduced into or generated in the collision cell 13 within a predetermined period of time can be compressed for mass spectrometry. This increases the amount of ions to be subjected to the mass spectrometry and correspondingly improves the detection sensitivity.
However, in this method, the ions which have been almost simultaneously discharged from the collision cell 13 are dispersed in the travelling direction according to their mass-to-charge ratios during their travel to the orthogonal accelerator 16. Since the OA-TOFMS and the collision cell are normally placed in different vacuum chambers separated from each other by a partition wall, the travel path from the collision cell 13 to the orthogonal accelerator 16 is comparatively long. Therefore, at the timing of the acceleration by the orthogonal accelerator 16, the ions are distributed in an elongated form along their travelling direction according to their mass-to-charge ratios, and only the ions falling within a specific mass-to-charge-ratio range will be ejected toward the flight space. Consequently, the ions falling within the specific mass-to-charge-ratio range are detected with a high level of sensitivity, while the ions outside that range cannot be observed. In the device described in Patent literature 1, the delay time mentioned earlier can be changed to adjust the range of mass-to-charge ratios which can be observed. However, when a mass spectrum covering a wide range of mass-to-charge ratios needs to be obtained, the device has problems, such as the measurement time being considerably long due to the necessity to perform the measurement multiple times while gradually changing the delay time.
In view of such a problem, various methods for achieving a high level of detection sensitivity for ions over a wide range of mass-to-charge ratios have been proposed so far.
For example, in a device described in Patent Literature 2, an area with a temporally changing electric field is provided in the previous stage of the OA-TOFMS. The temporal change of the electric field in this area is regulated to control the speed of the ions so that ions having different mass-to-charge ratios will be almost simultaneously introduced into the orthogonal accelerator.
In a device described in Patent Literature 3, an ion trap is placed in the previous stage of the OA-TOFMS. In the process of ejecting ions from the ion trap, the delay time mentioned earlier is continuously changed so that the amount of ions within a different range of mass-to-charge ratios will be increased at each ejection of the ions from the orthogonal accelerator.
In a device described in Patent Literature 4, an ion guide divided into a plurality of segments along the ion beam axis is provided in the previous stage of the OA-TOFMS. A different voltage is applied to each segment of the ion guide to make the ion guide function as an ion-accumulating unit as well as an ion-discharging unit. When ions accumulated in the ion-accumulating unit are discharged, the voltages respectively applied to the ion-accumulating unit and the ion-discharging unit are controlled so that the same amount of kinetic energy will be imparted to each of the ions having different-mass-to-charge ratios, and ions having larger mass-to-charge ratios will be discharged earlier, with the result that the ions having different mass-to-charge ratios will be almost simultaneously introduced into the orthogonal accelerator.
In a device described in Patent Literature 5, an ion guide capable of accumulating ions is provided in the previous stage of the OA-TOFMS. The accumulated ions are gradually discharged in small amounts so that the ion having the smallest mass-to-charge ratio among the accumulated ions will be discharged first, and the orthogonal accelerator is operated to eject ions for each discharging step. At every ejection of the ions, the voltage applied to each section of the device and the timing of its application are adjusted for the ions sent into the orthogonal accelerator.
Any of those various conventional devices proposed thus far requires a special ion optical system to be added, or complex control to be performed, in order to accumulate ions or control the speed of ions. Such devices have the problem of being expensive or large in size.